1. Field of the Invention
The present invention relates to a susceptor and to a coating apparatus and coating method using the susceptor.
2. Background Art
Epitaxial growth techniques are used to manufacture semiconductor devices requiring a relatively thick crystalline coating or film, such as power devices, including IGBTs (Insulated Gate Bipolar Transistors).
In order to produce an epitaxial wafer having a considerable coating thickness with high yield, it is necessary to continuously bring a fresh supply of material gas into contact with the uniformly heated surface of the wafer and thereby increase the coating speed. To do this, it is common practice that the wafer is subjected to epitaxial growth while it is rotated at high speed (see, e.g., Japanese Laid-Open Patent Publication No. 5-152207 (1993)).
In the vapor deposition apparatus disclosed in the above publication, an annular susceptor for supporting the wafer is fitted into a susceptor support, and the rotary shaft coupled to the susceptor support is rotated to rotate the wafer. It should be noted that the inner side of the susceptor has a countersink to receive the peripheral portion of the wafer thereon. That is, only a very narrow peripheral portion of the bottom surface of the wafer is in contact with the susceptor, and the rest of the bottom surface directly faces the surface of the heat equalizing plate for heating the bottom surface of the wafer. This structure is disadvantageous in that the wafer might be contaminated with contaminants such as metal atoms originating from the heating unit and the rotary portion, resulting in degradation of the electrical characteristics of the formed epitaxial film.
Further, according to the above patent publication, the mixture of material gas and carrier gas introduced into the reaction chamber flows radially across the top surface of the wafer from the center portion to the peripheral portion of the top surface due to the centrifugal force generated by the rotation of the wafer, and eventually exits from the reaction chamber through the exhaust port. It has been found, however, that since the susceptor is annular in shape, part of the gas reaching the peripheral portion of the wafer flows through the gap between the peripheral portion and the susceptor to the opening of the susceptor, resulting in the formation of an epitaxial film between the wafer and the susceptor. This film acts to attach the wafer to the susceptor, which may cause a crystal defect called a “slip” in the wafer as well as hampering the transfer of the wafer. The slip acts to warp the wafer and generate a leakage in the IC device, thereby greatly reducing the yield of the IC device.
In order to solve this problem, a susceptor has been proposed which includes an annular first susceptor portion for supporting the peripheral portion of the wafer and a disc-like second susceptor portion closely fitted into the opening of the first susceptor portion. This susceptor can prevent the wafer from being contaminated by contaminants originating from the heating unit and the rotary portion, since the second susceptor portion completely stops up the opening of the first susceptor portion. Furthermore, it is possible to block the flow of mixed gas through the gap between the peripheral portion of the wafer and the susceptor.
Incidentally, when the wafer is being mounted on the above susceptor, part of the mixed gas is compressed between the wafer and the second susceptor portion. The increased pressure of this compressed mixed gas due to the weight of the wafer causes the wafer to be displaced from its predetermined position, although the gas eventually escapes from between the wafer and the second susceptor portion. Another problem arises when the bottom surface of the wafer is heated in order to form an epitaxial film on its top surface. That is, if the entire bottom surface of the wafer is in contact with the second susceptor portion, the wafer is warped concavely upward by the heat, which might prevent the film formation on the wafer while rotating the wafer.
To solve this problem, the peripheral portion of the wafer may be supported on the first susceptor portion, and a gap may be provided between the wafer and the second susceptor portion. This arrangement prevents displacement of the wafer from its predetermined position on the susceptor when the wafer is being mounted on the susceptor, as well as avoiding the problem of the wafer being warped concavely upward when it is heated. In this case, however, another problem arises, as described below.
FIG. 8 is a partial cross-sectional view schematically showing a peripheral portion of the wafer and the adjacent portion of the susceptor as described above. Referring to FIG. 8, a susceptor 302 includes a first susceptor portion 302a for supporting the peripheral portion of a silicon wafer 301 and a second susceptor portion 302b closely fitted into the opening of the first susceptor portion 302a. A gap 303 is provided between the silicon wafer 301 and the second susceptor portion 302b. 
The susceptor 302 is heated by a heater (not shown) situated below it (as viewed in FIG. 8), and the silicon wafer 301 is heated by the heat from the susceptor 302. At that time, the peripheral portion of the silicon wafer 301 is heated by the heat from the first susceptor portion 302a since they are in contact with each other. The rest of the silicon wafer 301, on the other hand, is heated by the heat transferred from the second susceptor portion 302b through the ambient gas in the gap 303. It should be noted that the peripheral portion of the silicon wafer 301 is heated to a higher temperature than the rest of the silicon wafer 301 since the first susceptor portion 302a is made of SiC and hence has a lower thermal resistance than the ambient gas in the gap 303. Consequently, the temperature distribution across the silicon wafer 301 is not uniform and, hence, the thickness of the formed epitaxial film is not uniform. Further, thermal stress is concentrated at the points of contact between the silicon wafer 301 and the first susceptor portion 302a, which may result in breakage of the susceptor 302 and the formation of slips in the wafer.
The present invention has been made in view of the foregoing problems. It is, therefore, an object of the present invention to provide a susceptor constructed to be effective in reducing at least one of; the attachment of the wafer to the susceptor, contamination of the wafer with metal, and displacement of the wafer, or ensuring a uniform temperature distribution across the wafer, or both.
Another object of the present invention is to provide a coating apparatus and coating method capable of forming a film having a uniform thickness.
Other challenges and advantages of the present invention are apparent from the following description.